Hi Alex, > -----Original Message----- > From: Alexander Graf [mailto:ag...@suse.de] > Sent: Monday, July 03, 2017 12:19 PM > To: Siva Durga Prasad Paladugu <siva...@xilinx.com>; Michal Simek > <michal.si...@xilinx.com> > Cc: u-boot@lists.denx.de; Tom Rini <tr...@konsulko.com>; Simon Glass > <s...@chromium.org> > Subject: Re: [U-Boot] [PATCH] arch: armv8: Remove the error when dcache is > off > > > > On 03.07.17 08:29, Siva Durga Prasad Paladugu wrote: > > Hi Alex, > > > >> -----Original Message----- > >> From: Alexander Graf [mailto:ag...@suse.de] > >> Sent: Thursday, June 29, 2017 7:34 PM > >> To: Siva Durga Prasad Paladugu <siva...@xilinx.com>; Michal Simek > >> <michal.si...@xilinx.com> > >> Cc: u-boot@lists.denx.de; Tom Rini <tr...@konsulko.com>; Simon Glass > >> <s...@chromium.org> > >> Subject: Re: [U-Boot] [PATCH] arch: armv8: Remove the error when > >> dcache is off > >> > >> > >> > >> On 29.06.17 15:44, Siva Durga Prasad Paladugu wrote: > >>> Hi, > >>> > >>>> -----Original Message----- > >>>> From: Alexander Graf [mailto:ag...@suse.de] > >>>> Sent: Tuesday, June 27, 2017 6:08 PM > >>>> To: Michal Simek <michal.si...@xilinx.com> > >>>> Cc: Siva Durga Prasad Paladugu <siva...@xilinx.com>; > >>>> u-boot@lists.denx.de; Tom Rini <tr...@konsulko.com>; Simon Glass > >>>> <s...@chromium.org>; Siva Durga Prasad Paladugu > <siva...@xilinx.com> > >>>> Subject: Re: [U-Boot] [PATCH] arch: armv8: Remove the error when > >>>> dcache is off > >>>> > >>>> > >>>> > >>>>> Am 27.06.2017 um 13:52 schrieb Michal Simek > >> <michal.si...@xilinx.com>: > >>>>> > >>>>>> On 27.6.2017 13:46, Alexander Graf wrote: > >>>>>> > >>>>>> > >>>>>>> On 27.06.17 13:20, Michal Simek wrote: > >>>>>>> Hi, > >>>>>>> > >>>>>>>> On 27.6.2017 13:01, Alexander Graf wrote: > >>>>>>>> I don't think that's going to work - at least not without > >>>>>>>> compiler flag changes. By default, gcc will happily generate > >>>>>>>> unaligned accesses. If you disable dcache, these will trap. > >>>>>>> > >>>>>>> What's that compiler flags we should be using to avoid that? > >>>>>> > >>>>>> It's a combination of > >>>>>> > >>>>>> -mstrict-align > >>>>>> > >>>>>> plus crossing fingers with lots of praying plus making sure that > >>>>>> every code you call also follows -mstrict-align plus > >>>>>> double-checking that you don't break the kernel booting ABI: > >>>>>> > >>>>>> > >>>>>> http://elixir.free-electrons.com/linux/v4.12-rc7/source/Documenta > >>>>>> ti > >>>>>> on > >>>>>> /arm64/booting.txt > >>>>>> > >>>>>> > >>>>>> In the booti case, disabling dcache seems to be legitimate. In > >>>>>> the bootefi case however, it's not. > >>>>> > >>>>> Non wants to boot the kernel. It is really about programming stuff. > >>>>> > >>>>>> > >>>>>> So you will also need to set CONFIG_EFI_LOADER to depend on > >>>>>> !CONFIG_SYS_DCACHE_OFF which means you will want to convert > >>>>>> CONFIG_SYS_DCACHE_OFF to Kconfig first :). > >>>>> > >>>>> ok. I will let Siva to do it just wanted to refresh this topic. > >>>>> > >>>>> > >>>>>>> The reason for this change is to have really small u-boot which > >>>>>>> fits to OCM without DDR to be able to do initial programming. > >>>>>> > >>>>>> Yup, makes sense. I'm just slightly scared by the idea :). > >>> > >>> I can see that, we are anyway avoiding that error message for SPL > >>> BUILD where dcache is off. > >>> Also, dcache will be ON by default, until and unless user disable it > >>> through CONFIG_SYS_DCACHE_OFF. > >>> > >>> -mstrict-align is already used for armv8 in > >>> arch/arm/cpu/armv8/config.mk > >>> > >>> Finally I feel, we can remove this check or atleast we should make > >>> it as > >> #warning instead of #error. > >> > >> I put it in because UEFI binaries can assume that unaligned > >> instructions work. So please at least convert CONFIG_SYS_DCACHE_OFF > >> to Kconfig and make it mutually exclusive to CONFIG_EFI_LOADER. > > > > I got your point but, Iam bit concerned to do this change as it may > > break for someone if they are using both DCACHE_OFF and EFI_LOADER, > Isn't it? > > Today you can not use DCACHE_OFF at all, so I don't think it'd be a > regression ;). I want to make sure that we prohibit the combination on the > Kconfig level already to make sure that we do not break anyone using > DCACHE_OFF if we start to allow it. > > > I don’t have idea on UEFI side, all assume unaligned instructions work or it > is configurable when building? > > With UEFI you run 3rd party binaries in U-Boot context. The UEFI spec > explicitly states that unaligned accesses are valid. So because we by > definition don't control the payloads, we can not break unaligned accesses > which DCACHE_OFF does.
Thanks for clarification, then I will go ahead and make the change as you suggested.(convert CONFIG_SYS_DCACHE_OFF to Kconfig and make it mutually exclusive to CONFIG_EFI_LOADER). Thanks, Siva > > > Alex _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de https://lists.denx.de/listinfo/u-boot
